Aquifers

Any geological formation with pore spaces completely filled with water which stores and readily transmits groundwater to wells and springs is called an aquifer (Greek = Aqua (water) + Ferre (to bear)). Water can be pumped out from the aquifer by drilling wells into it.

There are two major types of aquifers:

Unconfined aquifers

In these aquifers, the water table is exposed to the atmosphere through openings in the overlying soil layer/s.

Confined aquifers

These aquifers are isolated from the atmosphere and extend between two impermeable layers.

Geology

According to the earth processes that form them, Rocks are classified as Igneous, Sedimentary and Metamorphic.

Igneous rocks are formed by cooling or solidification of molten rock material. These rocks are fine-grained and hard with less capacity to store and transmit groundwater. They become good aquifers when weathered or posses cracks and fractures.

Sedimentary rocks are formed by compaction of sediments eroded and transported by agents like wind, water, glaciers, etc. The sediments are derived from the weathering of pre-existing igneous, sedimentary and/or metamorphic rocks. These are the most important rocks for a hydrogeologist as many of these have a great capacity to store and transmit large amounts of groundwater.

Metamorphic rocks are formed when pre-existing Igneous, sedimentary or metamorphic rocks deform and get metamorphosed under the influence of deforming agents like temperature, pressure and/or chemically active fluids. These are never formed close to the surface.